Short-term experimental acidification of a Welsh stream: toxicity of different forms of aluminium at low pH to fish and invertebrates.

A 24-hr dosing experiment was carried out in a soft-water stream in upland Wales during which four separate zones were created by the simultaneous addition of sulphuric acid, aluminium sulphate, and citric acid. An upstream control zone (A), an acid zone pH 4.9 (B), an aluminium and acid (total filterable aluminium 0.27 mg/L, pH 4.9) zone (C) and a downstream zone (D) of aluminium complexed with citrate at low pH (total filterable aluminium 0.23 mg/L, pH 4.9). Test species exposed in all zones were the invertebrates Gammarus pulex (L.), Baetis rhodani (Pict.), Ephemerella ignita (Poda) and the fish Salmo salar L., Salmo trutta L. and Cottus gobio L. Response criteria measured were mortality, feeding, effects of pretreatment, brief-exposure, and the ability of animals to recover. Minimal effects were observed in the control and acid zones whilst large mortalities and reduced feeding were recorded in the acid and aluminium zone. These effects were significantly reduced in the acid, aluminium and citrate zone due to complexation of aluminium with citrate.